Akt, also called protein kinase B, represents a subfamily of the serine/threonine kinase. Akt was first described as the cellular homologue of the product of the v-akt oncogene (Bellacosa et al. 1991), and it has three members, Akt1/PKBα, Akt2/PKBβ and Akt3/PKBγ (Cheng et al. 1992; Jones et al. 1991a; Jones et al. 1991b). Activation of Akt depends on the integrity of the pleckstrin homology (PH) domain, which mediates its membrane translocation, and on the phosphorylation of Thr308 in the activation loop and Ser473 (Konishi et al. 1995). Phosphoinositides, PtdIns-3,4-P2 and PtdIns-3,4,5-P3, produced by PI3 K bind directly to the PH domain of Akt, driving a conformational change in the molecule, which enables the activation loop of Akt to be phosphorylated by PDK1 at Thr308 (Datta et al. 1999). Full activation of Akt is also associated with phosphorylation of Ser473 within a C-terminal hydrophobic motif (Datta et al. 1999). Although the role of PDK1 in Thr308 phosphorylation is well established, the mechanism of Ser473 phosphorylation is controversial. A number of candidate enzymes responsible for this modification have been put forward, including integrin-linked kinase (Persad et al. 2001), PDK1 when in a complex with the kinase PRK2 (Wick et al. 2000), Akt itself, through autophosphorylation (Toker et al. 2000), DNA-dependent kinase (Feng et al. 2004), and the rictor-mTOR complex (Sarbassoy et al. 2005). The activity of Akt is negatively regulated by tumor suppressor PTEN, which is frequently mutated in human malignancy (Vazquez et al. 2000). PTEN encodes a dual-specificity protein and lipid phosphatase that reduces intracellular levels of PtdIns-3,4,5-P3 by converting them to PtdIns-4,5-P2, thereby inhibiting the PI3K/Akt pathway (Stambolic et al. 1998).
Akt phosphorylates and/or interacts with a number of molecules to exert its normal cellular functions, which include roles in cell proliferation, survival, migration and differentiation (Cheng et al. 2001). Many lines of evidence demonstrate that Akt is a critical player in the tumor development and progression. In addition, aberrant hyperactivation of Akt pathway has been detected in up to 50% all human tumors (Sun et al. 2001; Cheng et al. 1997) and is closely associated with chemoresistance (West et al. 2002). Therefore, Akt has been an attracting target for anti-cancer drug discovery (West et al. 2002).
In the last several years, through combinatorial chemistry, high-throughput and virtual screening, and traditional medicinal chemistry, a dozen inhibitors of the Akt pathway have been identified. Lipid-based inhibitors of Akt were the first to be developed, including perifosine (Kondapaka et al. 2003), PX-316 (Meuillet et al. 2004) and phosphatidylinositol ether lipid analogues (Castillo et al. 2004), which were designed to interact with the PH domain of Akt. In addition, several Akt antagonists have been identified using high-throughput screening of chemical libraries and rational design. These inhibitors include 9-methoxy-2-methylellipticinium acetate (Jin et al. 2004), the indazole-pyridine A-443654 (Luo et al. 2005), isoform-specific allosteric kinase inhibitors (Lindsley et al. 2005) and Akt/PKB signaling inhibitor-2 (API-2), also called triciribine/TCN (Yang et al. 2004). API-2/TCN is a tricyclic nucleoside that previously showed antitumor activity in phase I and phase II trials conducted, but multiple toxicities, including hepatotoxicity, hyperglycemia, thrombocytopenia, and hypertriglyceridemia, precluded further development (Feun et al. 1993; Hoffman et al. 1996). By screen of the NCI diversity set, we have previously shown that API-2 inhibit Akt kinase activity and stimulate apoptosis of xenografts of human cancer cells exhibiting high Akt activity (Yang et al. 2004). This finding has provided new interest in studying this drug and raises the possibility that lower doses may inhibit Akt and induce tumor cell apoptosis without the previously associated side effects (Yang et al. 2004; Cheng et al. 2005).